1. Field of the Invention
The present invention relates to an apparatus for judging whether a Lippmann hologram which records the interference fringes of signal light (object light) and reference light is genuine or not.
2. Description of the Related Art
Holograms are often used in the so-called security field in which the authenticity of an article, that is, a judgment as to whether the article is genuine or not, is desired because the holograms, which are three-dimensional visual images, are difficult to produce, and are difficult to replicate. For example, the holograms are used to prevent counterfeiting of credit cards, ID cards, and cash vouchers such as gift vouchers. At present, surface relief-type rainbow holograms and embossed holograms using relief patterns on a reflecting surface (a front surface), the embossed holograms produced by the microfabrication technique adopting electron beam drawing, are often used. The embossed holograms are sometimes called relief holograms.
As the embossed holograms are thus widely used and become widespread, embossed hologram production equipment also becomes widely available, which sometimes results in the mass production of counterfeits. Since the principles of the embossing method use the relief patterns on the reflecting surface, a technique of exposing the relief pattern surface by ingeniously taking apart a hologram and transferring the relief patterns is also used as a method of counterfeiting.
As can be expected from the fact that there is the above-mentioned technique of transferring the relief patterns, the embossed holograms have the problem of being easily counterfeited. By contrast, it is extremely difficult to counterfeit Lippmann holograms which record the interference fringes as the difference in index of refraction inside a recording material. The reason is that advanced techniques are necessary to produce an image and recording materials are difficult to obtain. As a method of producing the Lippmann holograms, there are a photographed hologram illuminating a subject with a laser and a holographic stereogram recording on the basis of a multiple-view parallax image.
The process of producing the Lippmann holographic stereogram is schematically made up of the content creation process including the acquisition of an image and processing such as editing of the acquired image, the hologram original plate production process, and the replication (mass-production) process. The image is acquired by imaging or computer graphics. Each of a plurality of images obtained in the image editing process is converted into a strip parallax image by a cylindrical lens, for example. The interference fringes of the object light of the image and the reference light are sequentially recorded on a hologram recording medium as strip element holograms, whereby an original plate is produced. The hologram recording medium is brought into intimate contact with the original plate, and is illuminated with a laser beam, whereby a hologram is replicated.
In this hologram, image information obtained by sequentially taking the image from different observation points in the lateral direction, for example, is sequentially recorded in the lateral direction as strip element holograms. When the observer views this hologram with both eyes, two-dimensional images in the right and left eyes are slightly different, causing a parallax in the eyes of the observer and thereby reconstructing a three-dimensional image.
As described above, when the strip element holograms are sequentially recorded, an HPO (horizontal parallax only) holographic stereogram having a parallax in only a horizontal direction is produced. With the HPO holographic stereogram, printing takes a shorter time, making it possible to realize recording with high image quality. Furthermore, it is also possible to create a vertical parallax in the recording method. The holograms having a parallax in both horizontal and perpendicular directions are referred to as FP (full parallax) holograms. Incidentally, in the FP holograms, each of a plurality of images obtained in the image editing process is converted into a rectangular parallax image by a spherical lens, for example. The interference fringes of the object light of the image and the reference light are sequentially recorded on a hologram recording medium as rectangular element holograms, whereby an original plate is produced.
Compared to the embossed holograms, the Lippmann holograms are difficult to counterfeit because the hologram information is recorded as the difference in index of refraction inside the recording material, and are suitable for a judgment as to whether an article such as a credit card or an ID card is genuine or not. Actually, counterfeiting of the embossed holograms has occurred. In the future, the Lippmann holograms are expected to be used for security purposes.
However, as in the case of counterfeiting of the embossed holograms, counterfeiters are trying various methods to counterfeit the Lippmann holograms as sophisticatedly as possible and at low cost. These methods include a method of creating an embossed hologram having the same design (image content) as a Lippmann hologram. Although a person who has knowledge of holograms can often find successfully that the embossed hologram created by this method is a counterfeit, there is a possibility that a person who does not have knowledge of holograms is not able to judge whether it is a counterfeit or not.
In the past, an apparatus illuminating a hologram at a predetermined angle for visual observation of the quality of the hologram and an apparatus detecting light with a light receiving element, the light reconstructed as a result of a hologram being illuminated at a predetermined angle, to judge whether the hologram is genuine or not have been proposed.
For example, Japanese Unexamined Utility Model Publication No. 63-198075 describes an apparatus which allows a hologram to be observed under an optimum condition by setting an optimum angle which incoming light forms with the hologram.
Japanese Unexamined Patent Application Publication No. 62-093754 describes that information for determination of authenticity is recorded on a card in advance as a hologram so as to be read differently according to the angle of the reference light, and the authenticity is determined by reading this information by the reference light at a predetermined angle.
Japanese Unexamined Patent Application Publication No. 2001-307171 describes an apparatus for judging whether a hologram is genuine or not, the apparatus provided with an incident direction changing unit changing the incident direction of measurement light for a hologram.
Japanese Unexamined Patent Application Publication No. 2007-147850 describes a hologram reader in which a point light source array of light is disposed, the hologram reader which can selectively turn on any point light source of the point light source array.
Japanese Unexamined Patent Application Publication No. 2002-333817 describes an apparatus for judging whether a hologram is genuine or not, the apparatus provided with a light source emitting light of a wavelength which meets the reconstruction condition of the hologram and a light source emitting light of a different wavelength and illuminating the entire hologram.